Adding an implementation for read/write locks into the pthread library.
These locks are writer priority, such that if any writers come in they
are given priority for writing.

Disable serial console on stm32f103-minimum usbnsh example project config. Devices enumerate after this change.

Approved-by: Gregory Nutt <gnutt@nuttx.org>

Disable serial console on stm32f103-minimum usbnsh example project config. Devices enumerate after this change.

6loWPAN: Back out part of the last commit.  That included two fixes for a problem.  One that didn't work and is unnecessary and one that is necessary.  The commit removes the former.

EFM32 I2C: Fix timeout calculation

Approved-by: Gregory Nutt <gnutt@nuttx.org>

New interface allows checking if RTC time has been set.  This allows to application to detect if RTC has valid time (after
reset) or should application attempt to get real time by other means (for example, by launching ntpclient or GPS).

According the Datasheet the WDD Value is the lower bound of a so called Forbidden Window and to disable this we have to set the WDD
Value greater than or equal to the WDV Value.  This seems to be a bug in the datasheet. It looks like we have to set it to a greater value than the WDV to realy disable this Thing.  When triggering the Watchdog faster than the (very slow) clock source of the Watchdog fires, this Forbidden Window Feature resets the System if WDD equals to WDV.

This Changeset disables the Forbidden Window by setting the WDD Value to the Maximum (0xfff) Value possible.

Combination of RXDMA + IFLOWCONTROL does not work as one might expect.
Since RXDMA uses circular DMA-buffer, DMA will always keep reading new
data from USART peripheral even if DMA buffer underruns. Thus this
combination only does following: RTS is asserted on USART setup and
deasserted on shutdown and does not perform actual RTS flow-control.

Data loss can be demonstrated by doing long up_mdelay inside irq
critical section and feeding data to RXDMA+IFLOWCONTROL UART.

 STM32F7: default CONFIG_STM32F7_DMACAPABLE to 'n'.  STM32F7 does not have CCM RAM but DTCM, so this option does not need to enabled. DTCM RAM is DMA-able through CPU AHBS bus.

tm32: stm32l15xxx_rcc: configure medium performance voltage range and zero wait-state when allowed by SYSCLK setting

Zero wait-state for flash can be configured when:
 Range 1 and SYSCLK <= 16 Mhz
 Range 2 and SYSCLK <= 8 Mhz
 Range 3 and SYSCLK <= 4.2 Mhz

Medium performance voltage range (1.5V) can be configured when SYSCLK is up to 16 Mhz and PLLVCO up to 48 Mhz.